As a method of producing thermoplastic resin sheet or film by quenching the thermoplastic resin material extruded through a T-die, a chill roll method and a water quenching method, for example, are known.
The chill roll method, however, has various disadvantages. For example, since during high-speed molding, air is caught between a roll and a molten resin sheet or the contact between the roll and the molten resin sheet is poor, unevenness in thickness and wrinkles are formed; and in the case of the production of, for example, thicker sheet, the difference in the degree of cooling between the obverse and reverse surfaces of the molten resin sheet causes curling, and insufficient quenching is responsible for a reduction in transparency or surface luster of the final product.
Also the water quenching method suffers from disadvantages in that insufficient or uneven cooling of a molten resin sheet, which is caused by swelling or swaying of the water surface due to a local radiation of the heat from the molten resin sheet on the surface of cooling water, gives rise to the formation of haze dots and uneveness in thickness and further to a reduction or unevenness in transparency and surface luster, and therefore high-speed molding cannot be conducted.
An advantage of the water quenching method over the chill roll method is that sheet or film can be cooled efficiently. For this reason, a method has been proposed to overcome the above-described problems of the water quenching method, in which the molten resin sheet is cooled by a cooling water stream flowing on both sides thereof. However, even in accordance with this improved method, properties such as transparency of the sheet or film and high-speed moldability are not sufficiently satisfactory. This tendency becomes noticeable as the thickness of the sheet or film is high or the molding rate is increased.
I have already developed a method of forming thermoplastic resin sheet or film, which overcomes the problems as described above (see Japanese Patent Application No. 55291/1982). Thus the present invention provides an improved method over the foregoing method.